US 6940828 B2 Abstract Apparatus, and an associated method, for facilitating communications in an OFDM communication system. A sending station of the OFDM communication system performs inverse discrete trigonometric transforms upon modulation symbols representative of data that is to be communicated. And, a transformed vector is formed by the inverse discrete trigonometric transformer. A symmetric extension adder adds an extension to the transformed vector to attain desired symmetry or anti symmetry properties. The apparatus formed at a receiving station of the OFDM system includes a discrete trigonometric transformer for performing transform operations upon data received thereat.
Claims(12) 1. Apparatus for a communication system having a sending station that utilizes OFDM (Orthogonal Frequency Duplex Multiplexing) techniques to communicate data, the data formatted into successive data vectors, said apparatus for facilitating formation of an OFDM (Orthogonal Frequency Division Multiplexing) symbol, said apparatus comprising:
an inverse discrete trigonometric transformer formed of both an inverse discrete cosine transformer and an inverse discrete sine transformer, the inverse discrete cosine transformer coupled to receive a first portion of each of the successive ones of the data vectors into which the data to be sent by the sending station is formatted, and the inverse discrete sine transformer coupled to receive a second portion of each of the successive ones of the data vectors; said inverse discrete trigonometric transformed for transforming values of the successive ones of the data vectors into inverse-transformed form, as transformed values, the transformed values used to form OFDM symbols.
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a discrete trigonometric transformer embodied at the receiving station and coupled to receive representatives of the OFDM symbols formed at the sending station and sent upon the communication channel to the receiving station, said discrete trigonometric transformer for transforming the OFDM symbols into nontransformed form.
11. The apparatus of
12. Apparatus for a communication system having a sending station that utilizes OFDM (Orthogonal Frequency Duplex Multiplexing) techniques to communicate data upon a communication channel, the data formatted into data vectors, said apparatus for facilitating formation of an OFDM (Orthogonal Frequency Division Multiplexing) symbol, said apparatus comprising:
an inverse discrete trigonometric transformer coupled to receive successive ones of the data vectors into which the data to be sent by the sending station is formatted, said inverse discrete trigonometric transformed for performing one of a first inverse discrete trigonometric transform and at least a second inverse discrete trigonometric transform for transforming values of the successive ones of the data vectors into inverse-transformed form, as transformed values, the transformed values used to form OFDM symbols, performance of the one of the first and at least second inverse discrete trigonometric transforms, respectively, made responsive to communication conditions upon the communication channel.
Description The present invention relates generally to a manner by which to facilitate communications in a communication system, such as a WLAN (wireless local area network) or fourth-generation (4G) cellular communication system, that utilizes OFDM (orthogonal frequency division multiplexing) techniques. More particularly, the present invention relates to apparatus and an associated method, for transforming data, that is to be communicated during operation of the communication system, through the use of discrete trigonometric transform techniques. By using trigonometric transform techniques, the conventional need always to transmit data in quadrature is obviated. Single side band (SSB) modulation of the data can, e.g., be utilized. And, communication performance can be improved relative to the use of conventional Fourier transform techniques, when data is to be communicated upon communication channels that exhibit certain channel conditions. And, a selected discrete trigonometric transform is selected to be used, depending upon channel conditions, to best communicate data pursuant to a communication service. Communication of data is a necessary adjunct of modem society. Data is communicated to effectuate many different types of communication services. Data communication systems are provided through which to effectuate the communication of the communication service. A communication system is formed, at a minimum, of a sending station and a receiving station, connected theretogether by way of a communication channel. Data that is to be communicated by the sending station is converted, if necessary, at the sending station into a form to permit its communication upon the communication channel. The receiving station operates to detect the data communicated upon the communication channel. Once detected at the receiving station, the informational content of the data is recovered. Many different types of communication systems have been developed and implemented. Different types of communication systems are utilized to effectuate different types of communication services. And, as advancements in communication technologies permit, improvements to existing communication systems are made, and new types of communication systems are developed. The advancements, generally, permit data to be communicated at increased throughput rates and in manners to increase the likelihood that the informational content of the data can be recovered at a receiving station in spite of poor communication channel conditions. A radio communication system is a type of communication system. In a radio communication system, communication channels are defined upon radio links formed between the sending and receiving stations of the communication system. Such communication channels, referred to as radio communication channels, do not require that a wireline connection interconnect the sending and receiving stations. Construction of the infrastructure of a radio communication system, as a result, is generally less costly than the corresponding construction costs of the infrastructure of a conventional, wireline communication system. And, a radio communication system can be implemented as a mobile communication system as communications are effectuated by way of radio channels rather than fixed, wireline channels. Radio links, upon which the radio communication channels are defined, are formed upon a portion of the electromagnetic spectrum. The portion of the electromagnetic spectrum allocated for use by a particular radio communication system is typically band width-constrained. That is to say, the communication capacity of a radio communication system is limited by the amount of spectrum allocated to the communication system. The only manner by which to increase the communication capacity of such a system is to increase the efficiency by which to utilize the spectrum allocated to the communication system. Digital communication techniques permit increased efficiency of usage of the spectrum allocated to the communication system. First use of such techniques, therefore, in a communication capacity of a radio system generally can be increased. Typically, digital communication techniques involve the digitizing of the data that is to be communicated. When the data is digitized, the data becomes formed of digital bits. The digitized bits are sometimes formatted into sequences according to a formatting scheme to form packets or frames of data. The data, once formatted into the packets or frame, are communicated during discrete intervals upon a communication channel. Once delivered to a receiving station, the packets or frames are concatenated together, and the informational content of the data is recovered. Because the data can be communicated at discrete intervals, a circuit-switched connection need not be maintained between receiving communication station. Instead, packet-switched channels can be used in which two, or more, sending and receiving station pairs can share the same radio link or channel. Conventional local area networks (LANs) communicate packets of data to effectuate communications between sending and receiving stations defined therein. Wireless networks, operable in manners analogous to LANs, referred to as WLANs (wireless local area networks) have also been developed and also are utilized to communicate data upon radio channels defined upon radio links. A variant of an IEEE (Institute of Electrical and Electronic Engineers) 802.11 operating specification sets forth a wireless operating protocol that has been proposed as the operating standard for a WLAN. And, the operating specification has been proposed as the operating protocol by which to operate a so-called 4 At least one implementation of a WLAN anticipates, or otherwise uses, OFDM (orthogonal frequency division multiplexing) techniques. OFDM techniques effectively form a hybrid of a multi-carrier modulation (MCM) and frequency shift keying (FSK) modulation. Frequency-divided carrier frequencies are defined in an OFDM system. And, the carriers are selected to be orthogonal to one another, such as by separating the carriers by integer multiples of the inverses of symbol duration of parallel bit streams that are to be applied thereto. The orthogonal carriers are transmitted simultaneously, thereby permitting an entire allocated channel to be occupied through an aggregated sum of narrow, orthogonal sub-bands. Conventionally, OFDM techniques perform transformations between time-domain data and frequency-domain data through the use of a unitary transformation, namely the discrete Fourier transform (DFT). And, through use of such a DFT together in conjunction with the use of cyclic prefixes permits a receiving station to be of relatively simple construction in the presence of frequency-selective channel conditions. A DFT is, however, a complex transform. And, data that is transmitted in a communication system that utilizes OFDM techniques must be transmitted in quadrature. Single side band transmissions, for example, cannot be transmitted in a DFT-based OFDM system. Also, through the use of the DFT as a sole unitary transformation used in OFDM-based systems, the use of other transformation-types that might be more appropriate for particular communication channel conditions are, in existing systems, unavailable. While use of non-DFT, unitary transformations have been considered for use in various communication systems, the use of such transformations have not been considered in a manner that would limit the complexity required of equalization operations at a receiving station operable in such a system. If a manner could be provided by which to utilize non-DFT transformations in an OFDM communication system, improved communication performance would be possible. It is in light of this background information related to communication systems that utilize OFDM techniques that the significant improvements of the present invention have evolved. The present invention, accordingly, advantageously provides apparatus, and an associated method, by which to facilitate communications in a communication system, such as a WLAN or cellular communication system, that utilizes OFDM (orthogonal frequency division multiplexing) techniques. Through operation of an embodiment of the present invention, a manner is provided for transforming data, that is to be communicated during operation of the communication system, through the use of discrete trigonometric transform techniques. Non-DFT (non-discrete Fourier transform) techniques, such as trigonometric transform techniques, are used. The conventional need otherwise always to transmit data in quadrature is obviated. Single side band (SSB) modulation of the data can, for example, be utilized. Communication performance, such as data throughput rates, can be improved, relative to communication systems that require the use of discrete Fourier transform techniques. And, by permitting selection of a particular discrete trigonometric transform, selection of the transform is matched with the communication channel conditions so that the communication performance of the sending and receiving station are optimized. In one aspect of the present invention, an inverse discrete trigonometric transform is provided for a sending station of an OFDM communication system. Data that is to be communicated by the OFDM sending station is formatted into data vectors. Successive ones of the data vectors are applied to the inverse discrete trigonometric transformer to be transformed thereat. Each data vector is formed of modulation symbols that are applied, in parallel, to the inverse discrete trigonometric transform. Transformed values are generated by the inverse discrete trigonometric transformer, also in parallel as a transformed vector. In another aspect of the present invention, symmetric extensions are added to the transformed vectors formed by the inverse discrete trigonometric transformer. The extensions are added to the transformed vectors to achieve desired cyclic shift properties for the trigonometric transform. Subsequent to addition of the symmetric extension, guard intervals are added, and the transformed vector, modified by the symmetric extension, is converted into serial form. In another aspect of the present invention, apparatus is provided for a receiving station operable in an OFDM communication system. The receiving station includes a discrete trigonometric transformer for transforming OFDM symbols applied thereto. The discrete trigonometric transformer is matched to the inverse discrete trigonometric transformer that forms part of the sending station of the OFDM system. When delivered to the receiving station, the serially delivered OFDM symbols are converted into vector form and are then applied to the discrete trigonometric transformer. In another aspect of the present invention, the inverse discrete trigonometric transformer, and the matched, discrete trigonometric transformer, are formed alternately, or in combination, an inverse discrete cosine transformer and an inverse discrete sine transformer. The symmetric extension is added by the symmetric extension adder adds extensions corresponding to which of the transformer-types is utilized. When both inverse discrete cosine and inverse discrete sine transformers are utilized, modified vectors form responsive to operation thereof are summed together prior to communication by the sending station to the receiving station. In one implementation, apparatus is provided for a WLAN (wireless local area network). The apparatus includes an inverse discrete trigonometric transformer formed at an OFDM sending station and a discrete trigonometric transformer formed at an OFDM receiving station of the WLAN. In these and other aspects, therefore, apparatus, and an associated method, is provided for a communication system having a sending station that utilizes OFDM (orthogonal frequency duplex multiplexing) techniques to communicate data. The data is formatted into data vectors. Formation of OFDM symbols is facilitated. An inverse discrete trigonometric transformer is coupled to receive successive ones of the data vectors into which the data to be sent by the sending station is formatted. The inverse discrete trigonometric transformer transforms values of the successive ones of the data vectors into inverse-transformed form as transformed values. The transformed values are used to form OFDM symbols. A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings that are briefly summarized below. The following detailed description of the presently-preferred embodiments of the invention, and the appended claims. Referring first to The communication system, in the exemplary implementation, forms a WLAN (wireless local area network) constructed to be operable generally pursuant to a variant of the IEEE 802.11 communication standard. Here, OFDM (orthogonal frequency division multiplexing) communication techniques are utilized. While the following description of the exemplary implementations of the present invention shall describe operation of the present invention with respect to its exemplary implementation in which the communication system is formed of a WLAN, other communication systems can analogously be represented. For instance, the communication system in which an embodiment of the present invention is implemented can form a cellular communication system having characteristics associated with a proposed fourth-generation (4G) communication system, such as a system proposed pursuant to an IETF standards-creating task force. Description of an embodiment of the present invention, accordingly, is analogous to that described below with respect to its implementation in the WLAN. The network part of the WLAN includes a plurality of spaced-apart access points (APs). A single access point The access point Forward link channels are defined upon the radio link More particularly, the mobile station and the access point each include radio transceiver circuitry capable of both sending and receiving data upon the forward and reverse link channels defined upon the orthogonal subscarriers of the radio link Here, the data that is to be communicated by the access point to the mobile station is formed on the lines In one implementation, the lines Modified, transformed vectors are formed by the symmetric extension adder The mobile station operates to detect the data, formed of OFDM symbols, communicated thereto upon the OFDM subcarriers. The mobile station includes antenna circuitry In contrast to conventional sending and receiving stations, the need to utilize discrete Fourier transforming techniques is obviated. Instead, non-DFT unitary transformations are used in OFDM systems that employ cyclic prefixes. These non-DFT unitary transformations, i.e., sinusoidal transformations or discrete trigonometric transformations, exhibit properties similar to discrete Fourier transforms that make use of cyclic prefixes when the input vectors applied to such transforming elements employ a form of symmetry or anti-symmetry. The sinusoidal transforms employ real-valued basis functions in contrast to complex valued basis functions that are used pursuant to discrete Fourier transformations. Because of this, single-side band (SSB) transmission is permitted through the use of sinusoidal transforms. When a communication system is significantly band width-limited, SSB transmissions advantageously reduce band width requirements. Use of the discrete trigonometric transforms can also be advantageous when data is communicated upon communication channels that exhibit intersymbol interference (ISI). Namely, the use of the discrete trigonometric transforms improves communication performance, relative to use of conventional DFT transforms, when the guard intervals between OFDM symbols is not great enough to capture the delay spreads of the channel upon which the data is communicated. Even though symmetric or anti-symmetric extensions are required to be added to the data that is to be communicated to achieve cyclic shift properties of the transformed values, under the ISI conditions, the throughput rates that can be achieved are increased relative to the use of conventional DFT techniques. The modulated symbols, a The receive part of the mobile station receives the OFDM symbols, r(k), subsequent to their communication upon the forward-link channels of the radio link Subsequent to communication upon the forward-link channels, received values, r(k) are applied, by way of the lines The performance of an OFDM system, such as the system First, and as indicated by the block Then, and as indicated by the block Thereafter, and as indicated by the block Thereby, a manner is provided by which to facilitate communications in an OFDM communication system. Trigonometric transform techniques are utilized instead of Fourier transform techniques. Single side-band modulation techniques can therefore be used and, depending upon channel conditions upon the communication channel upon which OFDM symbols are communicated, communication performance of the communication system can be improved. The previous descriptions are of preferred examples for implementing the invention, and the scope of the invention should not necessarily be limited by this description. The scope of the present invention is defined by the following claims: Patent Citations
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